Assault-style weapon in deadly Tulsa medical facility shooting was bought that day, sources say

(CNN)An assault-style weapon found at the scene of Wednesday's deadly shooting in an Oklahoma medical building had been bought that day, according to three federal sources briefed on the investigation.

A gunman -- who was later found dead -- opened fire on the second floor of a medical building on the campus of Saint Francis Hospital in Tulsa on Wednesday afternoon, killing four people. Fewer than 10 others were injured, authorities said.

The assault-style weapon was an AR-15 style firearm, a source said. A different weapon -- a handgun -- was purchased on May 29, a source told CNN.

The two firearms -- a semiautomatic rifle and a semiautomatic pistol -- were both found at the scene, Tulsa police Capt. Richard Meulenberg told CNN earlier.

"It was just madness inside, with hundreds of rooms and hundreds of people trying to get out of the building," Meulenberg told CNN.  

The mass shooting is among the latest instances nationwide of first responders and civilians coming face-to-face with the threat of gun violence in public places. It comes more than two weeks after a racist assault at a supermarket in Buffalo, New York, and a bloody attack at a church in California; and eight days after a heartbreaking massacre at an elementary school in Uvalde, Texas.

The Tulsa shooting was one of the 233 mass shootings that have happened in the US this year, according to the Gun Violence Archive. CNN and the archive define a mass shooting as one in which at least four people are shot, excluding the shooter.

Law enforcement received a call just before 5 p.m. Wednesday about a person with a firearm at the Natalie Medical Building, a physicians' office facility on the Saint Francis Hospital campus, Tulsa police Deputy Chief Eric Dalgleish said at a news conference.

Responding officers who arrived within minutes "were hearing shots in the building, and that's what directed them to the second floor," Dalgleish said.

The gunman was found dead by police as they worked their way inside the building, Meulenberg said, and has not been publicly identified.

Police suspect the gunman's fatal wounds were self-inflicted. Two of the deceased were found in the same room as the gunman, the police captain said.